Foil-type switching elements, such as membrane switches, foil pressure sensors or similar ones, in general include at least two essentially elastic foil layers arranged at a certain distance to one another. This is, for example, achieved by means of a spacer which is arranged around the active region of the switching element and on which the two foil layers are adhered with their respective borders. In the active region of the switching element, different contact arrangements are applied on the foil layers, an electrical contact being established between the arrangements when the two foil layers are compressed, so that the switching element is triggered. When the pressure is released from the foil layers, these in turn are restored to their spaced position due to their elasticity and the electrical contact between the various contact arrangements is interrupted.
Such switching elements have a very good response characteristic which can moreover be adapted to the respective purpose of application by the design of the elastic foil layers. Such a switching element further has a very low assembly height and is in particular characterized by the multifarious possibilities of designing the command button. This makes such switching elements particular suitable for the use in fields where small structural dimensions and a flexible design of the command buttons is required.
Due to these properties, such switching elements have meanwhile found a wide application in almost all fields of engineering. A specific application of such switching elements relates to seat occupancy sensors in vehicles. Such seat occupancy sensors include a multitude of individual switching elements arranged across a seating surface of a seat in the vehicle seat. The switching elements are, for example, arranged between the seat foam and the seat cover.
Especially this location for employing the switching elements causes however problems for some seats. In some seats, in particular in case of leather fittings, the tension in the seat cover is indeed so high that some of the switching elements are already activated by the seat cover and are thus triggered even if the seat is not occupied. This is a so-called preload. Such a triggering caused by the position of assembly can be principally avoided by a corresponding design of the carrier foils, e. g. by using less flexible carrier foils having a higher modulus of elasticity. However, the use of less flexible carrier foils simultaneously results, in particular with pressure sensors, in a deterioration of the dynamics of the switching element.
From the U.S. Pat. No. 4,382,165, a foil-type switching element is known which can be employed in keyboard pads. The carrier foils of the switching element are separate and electrically insulated, by a spacer of dielectric material which is directly applied on one of the carrier foils with a predetermined pattern. The predetermined pattern comprises openings which enable a local electrical contact between contact surfaces applied on the two foils, the spacer essentially keeping the foils separate.
The U.S. Pat. No. 4,594,482 discloses an input pad having two transparent layers, at least one of which is flexible. Each layer carries an electrically conductive film on one of its surfaces. Both layers are disposed at a certain distance one to another such that the respective films are facing each other. A first spacer is located at the periphery of the thus attached layers for separating them. Additional punctiform spacers are distributed across the area of the input field and applied on one of the layers.